The present invention relates generally to linear amplifiers and particularly to fiber optic transmission links in which it is desirable to minimize linear distortion in high power RF signals.
Multi-channel broadband fiber optic transmission links using current-modulated laser diodes are limited in dynamic range by even and odd harmonic distortion products generated in response to frequency mixing products of the modulation signal. In particular, even and odd harmonic distortion limits multi-octave bandwidth performance, and third-order distortion limits sub-octave bandwidth performance. These non-linearities worsen as RF power to the laser diode is increased. As RF power to the laser diode is increased, clipping occurs when the laser diode is driven below the laser threshold current. Sub-threshold characteristics of laser diodes severely limit the useful dynamic range of fiber optic transmission links.
An optical signal transmission arrangement to reduce even harmonic distortion of a light emitting diode is described in U.S. Pat. No. 4,393,518 issued to Briley on Jul. 12, 1983. Briley divides an electrical input signal into a positive and a negative portion with respect to a selected reference level. These divided signals are transmitted independently through two optical transmitters and received by differential photodiodes to recover the original input signal. While this arrangement may reduce even harmonic frequency distortion, the light emitting diodes are operated only above the biasing point. Laser diode signal transmission systems, on the other hand, typically modulate the laser diodes above and below the biasing point.
Nazarthy et al., U.S. Pat. No. 5,253,309, issued on Oct. 12, 1993, discloses modulated optical transmission systems using two optical fibers to reduce second harmonic distortion, but does not compensate for signal clipping introduced by the electro-optical modulator""s nonlinear transfer function.
Piehler et al., U.S. Pat. No. 5,940,196, issued on Aug. 17, 1999, discloses an optical transmission system that combines multiple signals having different wavelengths and identical modulation to increase signal-to-noise ratio, but does not compensate for signal clipping introduced by the electro-optical modulator""s nonlinear transfer function.
A dynamic range extender for optical transmitters of the present invention comprises a bipolar distortion compensator for increasing drive signal gain as the absolute level of an input signal increases beyond a selected input voltage threshold, a signal coupler for dividing the input signal into complementary signals, a unipolar distortion compensator for increasing drive signal gain of each complementary signal beyond a selected forward current threshold, a signal clipper for pre-clipping each complementary signal below a selected clipping threshold, and complementary driver outputs to drive each of a pair of laser diodes in a complementary push-pull arrangement. The pre-clipping prevents the laser diodes from being driven below their threshold current level, and the distortion compensation suppresses second and third order harmonic distortion when the complementary signals generated by the laser diodes are combined by differential photodiodes.
An advantage of the dynamic range extender for optical transmitters of the-present invention is that high fidelity RF signals may be generated from laser diodes at high optical power levels.
Another advantage is that RF optical power levels of currently available laser diodes may be extended beyond their linear operating range while maintaining low harmonic distortion.
Still another advantage is that present invention may substantially increase the number of communication channels in parallel optical channel applications.
Yet another advantage is that the transmission range of a communications signal may be extended without sacrificing fidelity, reducing the number of repeaters and amplifiers required for multiple subscriber reception.